The use of dichroic dyes in solution with various liquid crystal materials and use of these so-called "guest-host" mixtures in liquid crystal devices, e.g., liquid crystal displays, are well known. In liquid crystal displays, the host liquid crystal material has its orientation controlled by the application of electric fields which in turn interacts with the guest dichroic dye to induce a cooperative conforming alignment. The dichroic dyes manifest low or high absorbance of incident light depending upon their molecular orientation with respect to the light wave vector. Generally, when an electric field is applied to the display, the area between the electrodes appears clear, i.e., all the dyes exhibit minimum absorption, and in the absence of an electric field the display appears dye colored, i.e., the dyes are in a high absorbing state.
Dyes suitable for such guest-host mixtures must have high order parameters and proper solubility in the host liquid crystal material, and the guest-host mixtures should provide good viewing characteristics (e.g., high contrast), high extinction, and chemical and photochemical stability.
Several patents describe dichroic azo dyes that have N,N'-unsubstituted dihydroperimidines as end group moieties. For example, Huffman et al, in U.S. Pat. No. 4,565,424, describe poly(arylazo) dyes containing a 2,3-dihydro-2,2-disubstituted perimidine moiety; Etzbach et al, in U.S. Pat. No. 4,667,020, describe diazo dyes containing a 2,3-dihydro-2,2-disubstituted perimidine moiety; and Cole, Jr. et al, in U.S. Pat. No. 4,122,027, describe a pentaazo dye containing a 2,3-dihydro-2,2-disubstituted perimidine moiety. In each of these cases the nitrogen atoms at the 1 and 3 positions of the dihydroperimidine are bound to a hydrogen atom.
Azo dyes containing dihydroperimidine moieties unsubstituted at the nitrogen positions exhibit relatively low solubility characteristics in nematic hosts. The N--H bonds in the dyes result in relatively high crystalline melting points and thus solubility is reduced. Shannon, in U.S. patent application Ser. No. 07/375,166, filed Jul. 3, 1989, discloses that azo dyes having N,N'-disubstituted-2,3-dihydro perimidine end groups, particularly those with alkyl substitution at the 2-position, are more soluble than the same dyes having N--H bonds, and that guest-host mixtures prepared with such dyes have excellent properties, most notably improved contrast.
It is often desired to provide a liquid crystal display which appears neutral black in the absence of an electric field and clear or non-black in the presence of an electric field. Generally, guest-host compositions used in such displays contain at least three dichroic dyes having their absorption maxima centered at approximately 450, 550 and 650 nm (in order to absorb strongly throughout the region between 400 and 700 nm). When using the dyes described by Shannon, supra, in such a formulation it is often necessary to use a dye having a large number of azo groups (i.e., tetra- or penta-azo dyes) to obtain chromophores with absorption at longer wavelengths. These dyes have lower solubilities in host liquid crystals than their shorter chain (fewer azo group) counterparts, making them unsatisfactory for certain guest-host formulations. The inventors have discovered that the wavelength of dye absorption can be shifted toward longer wavelengths by replacing one or more phenyl core radical of the dyes described by Shannon, supra, with a naphthyl or anthracyl radical. The resultant dyes have an appropriate wavelength for preparing black dye formulations and a higher solubility than their longer chain (larger number of azo groups) counterparts.